When your new mobile phone is delivered, the battery will not be fully charged. The instructions will almost certainly tell you to charge the battery for at least 14 hours continuously, or for even longer. You are well advised to do this for the second charge as well.

Yes, this is very frustrating, and will stop you trying your new purchase out, but please do follow the instructions. It can make a huge difference to the performance you'll get from the battery in the future.
Note that the phone will indicate that the battery is fully charged after an hour or two, but for the initial charge, you should ignore this.

Why is this necessary?

A battery is made of several cells wired in series. Although good quality batteries are made with matched cells, there will be some variation between the cells in any battery.

When you charge the battery, some of the cells will be fully charged first. The charge current has to pass through all of the cells, and you have to be sure that the trickle-charging completely fills every cell, even though some of the cells are "full" and therefore the charger control circuits are cutting back the charge current. The way to ensure this is to leave the battery on charge for a long, long time for the first couple of charges.

Establish a routine

Depending on the type of battery you have, and how you use your mobile phone, what you do to keep the battery charged and ready will vary. If you are able to establish a routine, it is much more difficult to end up with a flat battery in your phone when you need to make or take a call. You may prefer to leave the phone switched on all the time, and charge every other night, or to switch the phone off, and charge it once a week. Another method may suit you better.

Fill it up!

The way that most batteries charge, they are 50% full in just a few minutes, but it takes longer and longer to approach the 100% mark. Most chargers switch to trickle-charging at around 90% capacity (see below) but if you leave the battery on charge for longer, it will slowly fill right up. Most people find charging overnight the easiest way to do this.

Controlling the charge

Charging a battery is effectively turning electrical energy into chemical energy, which can be stored and converted back when electrical energy is needed to power the phone.

In general, the charging circuitry that regulates the charge current is built into the phone itself. It detects when the battery is fully charged, then cuts the charge current back to short pulses to keep it topped up. It does this by measuring the voltage of the battery as it is charged.

This voltage rises as the battery fills up, but the rate of rise of the voltage falls away at the point where it is full. Mathematicians recognise this as the delta of the charge voltage going negative, and this negative delta detection is what the charge control circuit uses to know the battery is full.

If the phone is unable to detect that the battery is full, the excess power (which cannot be turned into chemical energy) is turned into heat, and as a safety "back stop" the phone monitors the temperature of the battery, and stops the charging if it overheats. This could be too late, though...

Stay cool

The biggest killer of rechargeable mobile phone batteries is heat. This heat usually comes from overcharging. The charge controller may not be able to tell that the battery is fully charged, possibly because you took it off charge when full, switched it off then on again and then put it back on charge a short time later (maybe taking it out of and back into a car kit). If this happens, the "negative delta" point is already passed, and only the over-temperature limit is left. By the time this cuts in, the cells can have been damaged.

Discharge carefully

You can buy battery conditioners, which claim to be able to revitalise tired batteries by discharging and recharging the cells. they often fail to do this effectively, though. If you use a bulb and wire to discharge a battery, you will probably do more damage than good.

Why? and How?

The cells in a battery (for NiCd and NiMH each gives 1.2volts) are wired in series to give the power needed by the phone. Without being able to connect to each individual cell, a discharger can only guess at the state of the individual cells, and they generally just run the battery down till the total voltage adds up to 1v per cell. Professional dischargers connect directly to each individual cell, but few mobile phone batteries have the connections needed to do that.

If you simply use a bulb to drain the battery, you will run it completely flat. As mentioned above, batteries are made of cells wired in series, and some have more capacity than others.

As you discharge a battery, there will come a point when one cell is empty, but the others have charge remaining. If you continue beyond this point, you will start to reverse-charge the cell, which damages it, reducing its capacity.

Next time the battery is discharged, the weakest cell is even weaker, so the damage gets worse and worse, until you have no choice but to buy a new battery.

The best way to discharge a mobile phone battery is to leave it on the phone till the phone switches off.

Nickel Cadmium (NiCd)

All the foregoing applies to NiCd batteries. To get the best life out of a NiCd, let it run down every second or third charge. Do it more often and you shorten its overall life: do it less often, and you risk reducing its charge capacity.

Nickel Metal Hydride (NiMH)

NiMH batteries need much the same care as NiCd, except that you only need to run them down every week or two, if they are charged every night.

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Lithium Ion (Li-Ion)

Lithium Ion batteries are very different. You should not deliberately discharge a Li-Ion cell. In fact, if you were to manage to run one flat, it would probably be damaged. There is electronics inside each Li-Ion battery to protect it from such abuse, but don't take the risk!

To keep your Li-Ion battery in good shape, simply charge it overnight before it runs down. If a full battery at all times matters to you, you can top it up whenever you like, but you'll probably get a longer service life from it if you only recharge it when it is getting a bit low.

Storage

Batteries of any type don't like to be left discharged. In general, if you have a spare battery, it is probably best to use it alternately with its partner.

Batteries of any type don't like to be left discharged. In general, if you have a spare battery, it is probably best to use it alternately with its partner.

Declining years

Age and infirmity come to all of us, but mobile phone batteries get there quicker than their users!

NiCd

A NiCd battery will lose its charge capacity, and may run flat on its own. This is often caused by sharp, spiky crystals growing through the separators of the cell, causing a short circuit. It is possible to "flash" these away by applying a very high current (such as from a large battery) for a short while. The current through the spike will melt it away, curing the short circuit, but that's not really a cure: the hole in the insulator will still be there, and there will probably be other crystals poised to do the same in another place. If your NiCd battery has managed 700 or more charge cycles, or has been exposed to excessive heat or other abuse, replace it!

NiMH

A tired NiMH battery will probably give good standby times, but as soon as you make or receive a call, you'll discover that it can't provide the current needed. This is because age and heat cause the crystals inside the cell to get bigger, which means that their surface area falls in proportion to their volume. Unfortunately, there is nothing much you can do about this. If a NiMH battery has managed 500 or more charge cycles, it has done well. Time for a replacement!

Li-Ion

Li-Ion batteries can fail suddenly, possibly because the electronics inside it have gone wrong, but in general they simply fade away. Because the capacity falls gradually over the charge cycle life, when to replace it is a matter of when the charge capacity is no longer sufficient for your needs. Never try to revitalise a Li-Ion battery in any way, or expose it to excessive heat: the very high power density of Li-Ion makes such actions very dangerous.

Replacement

Because of the subsidy system, it is often cheaper to upgrade to a new model of phone (complete with new battery) than it is to buy a new battery. Having said that, it really is worth replacing a worn-out battery. It is common for people to remark that they wish they'd bought a new battery sooner - putting it off is rarely wise!

Disposal

When it is time to say good bye to an old, tired, battery, don't throw it on a fire: it could explode. Don't put it in your dustbin: there should be facilities for recycling rechargeable batteries provided by your local council.